This invention relates to a transmitter for use in transmitting an amplitude modulated radio frequency (rf) signal. The invention has been developed for use in a locator beacon which is being made the subject of an independent patent application and the invention is hereinafter described in the context of the locator beacon. However, it will be understood that the invention does have broader applications, to a transmitter for use in any rf communication system.
The transmitter has been developed to meet the requirements for a relatively low power demand and this virtually dictates the use of a Class C amplifier stage. However, the traditional problem with a Class C amplifier is that its modulation characteristic is non-linear and introduces distortion and this is incompatible with a requirement for low distortion transmission of audio information.
The present invention seeks to reconcile these conflicting requirements by providing a transmitter which comprises:
(a) an rf carrier signal generator for generating a rf carrier;
(b) a Class C amplifier arranged to amplify the rf carrier;
(c) a power supply for the Class C amplifier;
(d) a detector for detecting the modulated rf carrier and establishing a detected signal;
(e) a controller for controlling the power supply;
(f) means for supplying an audio signal to the controller for the purpose of amplitude modulating the rf carrier;
and wherein the audio signal and the detected signal are applied to the controller to compensate for the non-linearity of the Class C amplifier.
In a preferred embodiment, amplitude modulation sufficient for transmitting the audio form signal on the rf carrier can be achieved with reduced distortion while maintaining the low power demand advantage of the Class C amplifier.
Preferably, the detector comprises a directional coupler and/or a power detector.
Preferably, the power supply comprises a switch mode power supply.
Advantageously, the controller comprises an amplitude comparison means for performing an amplitude comparison between the audio signal and the detected signal.
In one embodiment, the transmitter further comprises a preamplification means between the rf carrier signal generator and the Class C amplifier for preamplifying the rf carrier.
Preferably, the transmitter further comprises an antenna for transmitting the amplitude modulating rf carrier, the antenna comprising a circuit board which constitutes an active portion of the length of the antenna and on which are mounted electrical components for tuning the antenna.